1. Field of the Inventions
This invention relates to a method of treating siliceous formations to increase the permeability of the formations.
2. Description of the Prior Arts
Subterranean sandstone or siliceous formations in oil and gas wells have been treated in the past with acid treatments to increase their permeability thus increasing or improving production from the formation. As used herein the term "siliceous" refers to the characteristic of having silica and/or silicate. Most sandstone formations are composed of over 70% sand quartz particles, i.e. silica (SiO.sub.2), bonded together by various amounts of cementing material including carbonate (calcite or CaCO.sub.3) and silicates. The acid treatment of siliceous formations should be distinguished from the acid treatment of carbonate formations. Carbonate formations can be treated with a variety of acid systems, including hydrochloric, acetic and formic acids, with usually equal success. The treatment of siliceous formations with these acids, however, has little or no effect because they do not react appreciably with the silica and silicates which characterizes the sandstone formations.
By far the most common method of treating sandstone formations involves introducing hydrofluoric acid into the wellbore and allowing the hydrofluoric acid to react with the surrounding formation. Hydrofluoric acid is preferred almost uniformly because of its reactivity with silica and silicates. The silicates include such things as clays and feldspars. Hydrofluoric acid tends to react very quickly with authigenic clays, such as smectite, kaolinite, illite and chlorite, especially at temperatures above 150.degree. F. Because of this, the hydrofluoric acid penetrates only a few inches of the formation before it is spent. Simultaneously, precipitation of various alumni and/or silicate complexes occur as a result of the reaction of the hydrofluoric acid with the clays and silts. The precipitation products plug pore spaces and reduce the porosity and permeability of the formation, thus impairing flow potential. Because clays are normally a part of the cementitious material that holds the sandgrains of sandstone formations together, the dissolution of clay also weakens and unconsolidates the sandstone matrix in the vicinity of the wellbore, thus causing damage to the formation. The damaging effects due to both the unconsolidation of the matrix and the precipitation of complexes which clog the pore spaces of the formation can eliminate or even revert the stimulation effect of the acid treatment.
One of the most common prior art methods of reducing the reaction rate of hydrofluoric acid with the area of the formation immediately surrounding the wellbore, is accomplished by the slow hydrolysis of ammonium bifluoride to convert it to hydrofluoric acid, either at the surface or within the well. While this method allows the acid to penetrate slightly further into the formation, it does not eliminate precipitates from forming and clogging the matrix. Other methods such as retarding the acid reaction by introducing an oil wetting surfactant into the well to prevent and reduce contact of the acid with the formation have also been used with limited success. Oil wetting surfactants create problems of their own which inhibit the production of oil and gas from the formation.
What is therefore needed is a method of treating siliceous or sandstone formations using hydrofluoric acid which reduces the amount of precipitates produced and which inhibits the reactivity of the hydrofluoric acid with the clay or silicate elements of the formation.